Modelling elements in conversion of solid fuels - fixed bed combustion and gaseous radiation

Doktorsavhandling, 2008

For a long heat and power production by combustion of solid fuels has time been an established technique. Environmental concern of emissions for NOx, SOx and other harmful species has caused an increasingly stricter legislation on the emission levels of the boilers. The rapidly growing awareness of climate changes due to the release of greenhouse-gases from the use of fossil fuels has resulted in additional challenges. To meet the emerging demands there is a need for further development of existing and new combustion techniques. The costs involved in building and development of boilers are large, and this makes tools supporting experience and test facilities extremely valuable for design and optimization of the equipment. One of these tools that have received attention the last decades is numerical simulation. Numerical modelling is intended to describe the physical mechanisms of the process and can give a fundamental understanding of the conversion in a boiler. To carry out the modelling a number of phenomena have to be considered: heat transfer, fluid dynamics, chemical reactions, and several other processes, dependent on application. As the computational cost often imposes a problem, approximations are introduced, and the related simplifications create uncertainties in the results. To obtain more reliable models, useful for quantitative predictions, more work is needed to evaluate the validity of model approximations. In this thesis, models in two areas, related to conversion of solid fuels, are investigated: fixed bed combustion and gaseous radiation. Combustion in fixed beds is of importance in grate furnaces and wood-fired domestic boilers. Modelling of combustion in fixed beds has here been examined with a one-dimensional model. The main aspect studied is the influence of gradients within individual fuel particles. The results show that these gradients only have a small effect on the ignition rate and maximum temperatures, while the effect on the distribution of the release of volatiles in the bed is more significant. Also the sensitivity of the bed model to a number of parameters related to heat transfer and reaction rates has been investigated, showing some sensitivity of the model to heat- and mass-transfer coefficients and the mixing rate of gaseous species. The work on gas radiation includes a theoretical and numerical investigation of various types of models accounting for spectral properties of gases and their relation to the solution of the radiative transfer equation. Further, spectral models have been examined for oxy-fired conditions, i.e. combustion in pure oxygen and recycled flue gas, a proposed method in relation to CO2 capture. Gas radiation modelling has been applied in propane and lignite flames as a tool to evaluate measured data.